(1) Field of the Invention
The present invention relates to an optical scanning device that performs exposure scanning of a photoreceptor to form an electrostatic latent image on the photoreceptor, and also relates to an image forming apparatus comprising the optical scanning device.
(2) Description of the Related Art
Typically, an image forming apparatus using electrophotography, such as a printer and a copier, (i) forms an electrostatic latent image corresponding to image data on a photoreceptor drum by causing an optical scanning device to perform exposure scanning of the photoreceptor drum, and (ii) forms a toner image by developing the formed electrostatic latent image by using toner. The formed toner image is transferred to a recording sheet, such as a recording paper and an OHP sheet. Then, a fixing device fixes the transferred toner image to the recording sheet.
An image forming apparatus capable of forming a full-color image comprises four photoreceptor drums to form four toner images in yellow (Y), magenta (M), cyan (C) and black (K), respectively. Electrostatic latent images are formed on the respective photoreceptor drums by the optical scanning device emitting laser beams that are in one-to-one correspondence with pieces of image data for Y, M, C and K and that irradiate the respective photoreceptor drums.
In the above image forming apparatus, the laser beams irradiating the four respective photoreceptor drums are each generated by a different one of four semiconductor laser elements. First, a first optical system causes the laser beams generated by the respective semiconductor laser elements to irradiate a rotating polygon mirror. Next, the rotating polygon mirror causes the four laser beams to proceed along a main scanning direction extending parallel to the direction of axes of the photoreceptor drums. Thereafter, a second optical system causes the four laser beams to irradiate the respective photoreceptor drums.
Each of the photoreceptor drums is rotated when irradiated by the corresponding laser beam. The laser beams scan the respective photoreceptor drums along the main scanning direction, while sequentially changing its position along a sub scanning direction. Consequently, an electrostatic latent image made of dots is formed on the circumferential surface of each photoreceptor drum.
In general, when a full-color image is to be formed, an electrostatic latent image having a high resolution of, for example, 1200 DPI is formed on each of the circumferential surfaces of the four photoreceptor drums. In contrast, when a monochrome image is to be formed, an electrostatic latent image having a low resolution of, for example, 600 DPI is formed only on the circumferential surface of one of the four photoreceptor drums on which a toner image in the color K is to be formed.
In such a case where the electrostatic latent image having a low resolution of 600 DPI is to be formed, a laser beam repeatedly scans one photoreceptor drum along the main scanning direction, with a distance of 42 μm between every two scan lines (along the sub scanning direction). In contrast, in a case where an electrostatic latent image having a high resolution of 1200 DPI is to be formed, each laser beam scans the corresponding photoreceptor drum along the main scanning direction, with a distance of 21 μm between every two scan lines (along the sub scanning direction). This is for increasing the dot density of the electrostatic latent image to be formed on each photoreceptor drum.
Therefore, in a case where an electrostatic latent image having a high resolution of 1200 DPI is to be formed, the number of times a laser beam scans a photoreceptor drum along the main scanning direction is twice the number of times a laser beam scans a photoreceptor drum along the main scanning direction in a case where an electrostatic latent image having a low resolution of 600 DPI is to be formed. Accordingly, the total number of rotation of the polygon mirror required for a single formation of a high-resolution electrostatic latent image is twice the total number of rotation of the polygon mirror required for a single formation of a low-resolution electrostatic latent image.
As such, in order to form a high-resolution electrostatic latent image, the total number of rotation of the polygon mirror should be twice the total number of rotation of the polygon mirror required for formation of a low-resolution electrostatic latent image. That is to say, in order to form a high-resolution electrostatic latent image as fast as a low-resolution electrostatic latent image, it is required to rotate the polygon mirror twice as fast as the rotation speed of the polygon mirror required for formation of the low-resolution electrostatic latent image. In general, however, the maximum rotation speed of the polygon mirror is approximately 50000 rpm. It is thus impossible to rotate the polygon mirror twice as fast as the rotation speed of the polygon mirror required for formation of the low-resolution electrostatic latent image. Hence, there is a limit on acceleration of the speed of forming a high-resolution electrostatic latent image.
Assume, for example, an image forming apparatus that forms an electrostatic latent image on the circumferential surface of a photoreceptor drum by causing one laser beam to scan the photoreceptor drum along the main scanning direction while changing the position of irradiation along the sub scanning direction. When forming an electrostatic latent image having a low resolution of 600 DPI, this image forming apparatus can perform a high-speed image forming operation by setting the polygon mirror to rotate at high speed (50 sheets per minute). However, when forming an electrostatic latent image having a high resolution of 1200 DPI on the photoreceptor drum, this image forming apparatus can only perform a low-speed image formation (25 sheets per minute).
Patent Literature 1 (JP Patent Application Publication No. 2008-26570) discloses an optical scanning device that forms an electrostatic latent image on each of paired photoreceptor drums by using a multi-beam method, according to which a plurality of semiconductor laser beams form optical spots on each of the paired photoreceptor drums. In this optical scanning device, two light source units are disposed while opposing each other with a polygon mirror therebetween, in order to irradiate each of the paired photoreceptor drums with a pair of optical spots.
In the optical scanning device having the above structure, each of the two light source units emits two laser beams that irradiate a corresponding one of the paired photoreceptor drums. If the two optical spots formed on each of the paired photoreceptor drums correspond to a high resolution of 1200 DPI, this optical scanning device can form an electrostatic latent image twice as fast as the speed of forming an electrostatic latent image using one laser beam per photoreceptor drum (for example, 50 sheets per minute).
An electrostatic latent image having a low resolution of 600 DPI can also be formed by using a light source unit having two semiconductor laser elements that generate two optical spots corresponding to a high resolution of 1200 DPI. In this case, only one of the two semiconductor laser elements provided in the light source unit may be driven to form an electrostatic latent image on a photoreceptor drum by using one laser beam.
However, in this case, an image forming operation is performed at the same speed as the speed of forming an electrostatic latent image having a high resolution of 1200 DPI—e.g., 50 sheets per minute. This makes it impossible to form a low-resolution electrostatic latent image faster than the speed of forming a high-resolution electrostatic latent image.
As described above, an image forming apparatus capable of forming a full-color image uses a light source unit corresponding to a high resolution, and thus can form a low-resolution monochrome image only at the same speed as the speed of forming a full-color image. For the above reason, such an image forming apparatus cannot satisfy the user's demand to form a monochrome image faster than the speed of forming a full-color image.
Furthermore, in the optical scanning device disclosed in Patent Literature 1, two light source units are disposed so as to oppose each other with a polygon mirror therebetween. This arrangement requires a large space on both sides of the polygon mirror. In addition, if four photoreceptor drums are disposed to form toner images in the colors Y, M, C and K, respectively, the optical scanning device disclosed in Patent Literature 1 must have two additional light source units, which may result in a significant increase in the size of the optical scanning device.
Patent Literature 2 (JP Patent Application Publication No. H11-14921) discloses an optical scanning device that forms electrostatic latent images having different resolutions on a single photoreceptor drum by switching between two light-emitting units each having a semiconductor laser array with a plurality of laser oscillators. In this optical scanning device, the two semiconductor laser arrays in the light-emitting units correspond to a low resolution and a high resolution, respectively. When an operation for forming a low-resolution image is performed, one of the light-emitting units that has the semiconductor laser array corresponding to a low resolution is used. This way, the operation for forming the low-resolution image can be performed faster than the speed of an operation for forming a high-resolution image.
The optical scanning device disclosed in Patent Literature 2 is associated with a monochrome image forming apparatus for forming two types of electrostatic latent images (i.e., a low-resolution electrostatic latent image and a high-resolution electrostatic latent image) on a single photoreceptor drum. This optical scanning device is structured such that a plurality of laser beams oscillated by the semiconductor laser array provided in each light-emitting unit (light source unit) irradiate a polygon mirror via a beam splitter. However, as a beam splitter is an expensive optical element, any structure incorporating a beam splitter is not economically efficient.
Furthermore, in order to apply the structure of Patent Literature 2, which relates to a monochrome image forming apparatus, to an image forming apparatus capable of forming a full-color image, the structure of Patent Literature 2 must be provided further with three additional photoreceptor drums and light-emitting units (light source units) that each form an electrostatic latent image having a high resolution on a corresponding one of the three photoreceptor drums. To achieve the above structure, the image forming apparatus disclosed in Patent Literature 2 needs to have a space for four light-emitting units corresponding to a high resolution and one light-emitting unit corresponding to a low resolution. This may increase the size of the image forming apparatus.
The following structure has been proposed for an optical scanning device in an image forming apparatus capable of forming a full-color image: as disclosed in Patent Literature 1, instead of disposing a plurality of light source units corresponding to a high resolution on both sides of a polygon mirror so they oppose each other with the polygon mirror therebetween, disposing four light source units on one side of a polygon mirror as disclosed in Patent Literature 1. In this case, the polygon mirror causes the laser beams that are emitted by the four respective light source units and that correspond to a high resolution to proceed along the main scanning direction of the four photoreceptor drums for forming toner images in the colors Y, M, C and K, respectively. Thereafter, these laser beams are reflected toward the four respective photoreceptor drums.
The problem with the optical scanning device having the above structure is that, since the four light source subunits are disposed in a relatively small space on one side of the polygon mirror while being adjacent to one another, it is difficult to secure a space for newly providing the semiconductor laser array corresponding to a low resolution and a beam splitter, which are disclosed in Patent Literature 2.
It is permissible to irradiate the polygon mirror with the laser beam emitted by the semiconductor laser array corresponding to a low resolution by, with use of the beam splitter, causing this laser beam to take the same optical path as the laser beam emitted by the semiconductor laser array corresponding to a high resolution. In this case, however, due to the limit on the positions of the semiconductor laser arrays and the beam splitter in relation to the polygon mirror, it is difficult to secure a space for providing these elements in a downsized optical scanning device. Furthermore, use of the expensive beam splitter reduces economic efficiency.